Local probe investigation of spin transport and dynamics in organic Semiconductors

Organic semiconductors fall into a class of materials that shows significant potential for future applications and as a result, the field is becoming extremely topical. This is due to their ease of processing, low cost, highly tuneable electronic properties, favourable mechanical properties and extremely long spin coherence times. The latter point makes them extremely promising for future spintronic applications. However, there is a lack of suitable techniques that can yield information on intrinsic spin and charge carrier dynamics in organic materials. For example, whilst there are experimental techniques available that can probe the spin polarisation of charge carriers conventional spintronic devices and materials, these have limitations (for example, they usually do not measure at buried interfaces) and are not typically applicable to organic materials. 

Low Energy Muon Spin Rotation can directly measure the depth resolved spin polarisation of charge carriers in organic spin injection devices [1]. Using this technique, it is possible to separate out the various contributions to spin decoherence, differentiating between interface and bulk spacer layer effects. It is possible to perform in-situ magnetoresistance measurements whilst at the same time measure the depth profile of the polarisation of injected electrons. One is thus able to correlate macroscopic and microscopic phenomena together, which gives information of fundamental importance to both the theoretical and device communities. An example of this is the ability to directly probe the effect of engineering interfaces [2]

Bulk muon techniques can also be used to probe the charge carrier and spin dynamics of organic semiconductors on a molecular lengthscale [3]. I will present measurements of temperature dependent electron spin relaxation rates, on a series of organic molecules of different morphology and molecular structure, which points towards some generality of the underlying spin scattering mechanisms in organic materials [4]. I will then present some of our latest results on the mass-dependence of electron spin relaxation rates, which offer clues as to the underlying relaxation mechanism [5]. 

[1] A. J. Drew et al., Nature Materials 8, 109 (2009)
[2] L. Schulz et al., Nature Materials 10, 39 (2011)
[3] A. J. Drew et al., Phys. Rev. Lett. 100, 116601 (2008)
[4] L. Schulz et al., submitted to Phys. Rev. Lett
[5] L. Nuccio et al., in preparation